html,
body {
    height: 100%;
    margin: 0;
}
.mainPage {
    display: grid;
    grid-template-columns: 200px auto;
    grid-template-rows: 40px auto;
    grid-template-areas: "header header"
    "left main";
    height: 100%;
}
.top{
    grid-area: header;
}
.menu{
    grid-area: left;
}
.main{
    grid-area: main;
}
iframe {
    margin: 0;
    width: 100%;
    height: 99%;
    border: 0;
}